In recent years, from the viewpoint of global environment conservation, the whole automobile industry aims to improve fuel efficiency of automobiles to regulate CO2 emissions. To improve the fuel efficiency of automobiles, reducing the weight of automobiles by using the components thereof with thinner walls is most effective. Therefore, in recent years, the usage of high strength steel sheet (highly strengthened steel sheet) as a material for automobile parts has increased.
Meanwhile, weldability of steel sheets tends to deteriorate as the strength increases. Therefore, a steel sheet with not only high strength, but also excellent weldability is desired. A steel sheet that does not have satisfactory weldability cannot be used for the automobile parts or the like, because it causes problems such as cracking when joined by welding. To reduce the weight of the automobile parts or the like, it is essential to develop a steel sheet that has both high strength and weldability, and various techniques have been proposed so far to provide high strength cold-rolled steel sheets and hot-dip coated steel sheets that focus on weldability.
For example, according to PTL 1, a high strength hot-dip galvanized steel sheet with excellent spot weldability, anti-crash property and bending formability can be obtained by containing, in mass %, C: 0.05% or more and 0.15% or less, Si: 0.01% or more and 1.00% or less, Mn: 1.5% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 0.50% or less, Cr: 0.010% or more and 2.000% or less, Nb: 0.005% or more and 0.100% or less, Ti: 0.005% or more and 0.100% or less, B: 0.0005% or more and 0.0050% or less, while containing Si, Mn, Cr and B in a specific content range, and having a metallographic structure (steel structure) including, in terms of an area ratio, ferrite: 10% or less, bainitic ferrite: 2% or more and 30% or less, and martensite: 60% or more and 98% or less, where the proportion of retained austenite determined by using an X-ray diffraction method is less than 2%, the proportion of massive martensite adjacent only to bainite in the whole metallographic structure is 10% or less, and a difference in hardness is specified between positions that are located 100 μm and 20 μm away from the surface.
According to PTL 2, a cold-rolled steel sheet with excellent spot weldability and with a tensile strength of 980 MPa or more can be obtained by containing, in mass %, C: 0.05% or more and 0.13% or less, Si: 0.05% or more and 2.0% or less, Mn: 1.5% or more and 4.0% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more and 0.1% or less, Cr: 0.05% or more and 1.0% or less, Nb: 0.010% or more and 0.070% or less, Ti: 0.005% or more and 0.040% or less, and N: 0.0005% or more and 0.0065% or less, wherein 70% or more of Ti in the steel is precipitated, and 15% or more Nb in the steel is left in a solute state.
According to PTL 3, a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an hot-dip galvannealed steel sheet with excellent ductility, stretch flange formability and weldability, having a tensile strength of 980 MPa or more, and 0.2% proof stress of 700 MPa or less can be obtained, while these contain, in mass %, C: 0.07% or more and 0.15% or less, Si: 1.1% or more and 1.6% or less, Mn: 2.0% or more and 2.8% or less, P: more than 0% and 0.015% or less, S: more than 0% and 0.005% or less, Al: 0.015% or more and 0.06% or less, Ti: 0.010% or more and 0.03% or less, and B: 0.0010% or more and 0.004% or less, and have a metallographic structure to be described below having, at a position located ¼ of the sheet thickness away from the surface of the steel sheet, an area ratio satisfying tempered martensite: 10 area % or more and less than 30 area %, bainite: more than 70 area %, a total of tempered martensite and bainite: 90 area % or more, ferrite: 0 area % or more and 5 area % or less, and retained austenite: 0 area % or more and 4 area % or less.